<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <button class="btn">获取</button><button class="btn">设置</button>
    <select class="sel" multiple="multiple">
        <option value="2001">2001</option>
        <option value="2002">2002</option>
        <option value="2003">2003</option>
        <option value="2004">2004</option>
        <option value="2005">2005</option>
        <option value="2006">2006</option>
        <option value="2007">2007</option>
        <option value="2008">2008</option>
        <option value="2009">2009</option>
        <option value="2010">2010</option>
        <option value="2011">2011</option>
        <option value="2012">2012</option>
        <option value="2013">2013</option>
        <option value="2014">2014</option>
        <option value="2015">2015</option>
        <option value="2016">2016</option>
        <option value="2017">2017</option>
        <option value="2018">2018</option>
        <option value="2019">2019</option>
        <option value="2020">2020</option>
    </select>
</body>
<script>
    var sel = document.querySelector('.sel')
    var getVal = document.getElementsByClassName('btn')[0]
    var setVal = document.getElementsByClassName('btn')[1]
    getVal.onclick = function () {
        console.log(sel.value)
        console.log(sel.selectedIndex)
    }
    setVal.onclick = function () {
        // sel.value = 2010
        sel.selectedIndex = 10
    }
</script>

</html>